1. Field of the Invention
This invention relates to a system and method for extending the dynamic range of an optical sensor, and more particularly to extending the dynamic range of an integrated circuit optical sensor of the type used in a Focal Plane Array (FPA).
2. Description of the Prior Art
A widely use prior art optical sensor is illustrated schematically in FIG. 1. It uses an optical to electrical converter, such as a photodiode 1, to generate a current proportional to the photon flux. A capacitor 3, serving as a linear integration well, collects this charge over a predetermined time interval established by a switch 4 operated by an integration control signal. The voltage across this integrating capacitor at the end of the interval is a function of the average intensity of illumination of the photodiode over the interval. A sample and hold circuit 5 samples the voltage at the end of the interval, and, in a typical system, an analog to digital converter 6 converts this analog voltage to a digital signal an transfers this digital signal to a processor 7. After each sample, the integrating capacitor is discharged, and the process is repeated. As will be appreciated by those skilled in the art, this technology is used in integrated circuit Focal Plane Arrays which use a plurality of these elements to make a two dimensional array of detectors from which an image is generated by the processor. While generally satisfactory, such arrays have limited dynamic range as a result of the necessarily limited size of the linear integrating capacitor in an integrated circuit implementation. This in turn limits the dynamic range of the array.
In certain applications a large dynamic range is required. For example, both bright and dim objects of interest may be present in the field of view or a moving object of interest may have both bright and dim portions.